ROCKET BIKEIt’s not every day we get a letter from a reader about a rocket-propelled bicycle–especially from a 12-year-old girl. Sarah Pickens’s letter really got our attention: “My dad and I have rocket bikes. My dad’s bike shoots out fire. Don’t worry, mine releases cold carbon dioxide instead, and goes about 30 mph.” Lucky kid. Turns out her father, Tim, is an engineer who designs rocket propulsion systems and worked on SpaceShipOne. Here’s how he built our favorite father-daughter project of all time (but we’d recommend readers leave this project to the pros):

I built the bikes to demonstrate how easy hybrid rockets are to operate, and I wanted to have a rocket propel me down the road. When I was working for [SpaceShipOne designer] Burt Rutan, I built this really nice bike. Basically, it uses the same type of rocket motor system we used on SpaceShipOne. The propellant is nitrous oxide–laughing gas–and the fuel is asphalt. Asphalt burns clean. It generates a brilliant flame, and can push the bike up to 60 mph in about 5 seconds. Sarah’s bike took 5 hours to build. Mine took longer because it burns propellants (I told Sarah she can’t have fire until she is 18).

Each bike has a 5-pound tank with a valve that connects to the handlebars and controls the throttle–on my bike the throttle controls up to 200 pounds of thrust. The more you squeeze the throttle, the more nitrous (or CO2) flows from the tank. We aren’t going for the Darwin Award. If my finger comes off the throttle, the motor will shut down. Essentially, my motor is a piece of pipe with asphalt inside. I stick a sparkler into the motor and squeeze the trigger when I see fire coming out. When I ride, I’m pulling almost a g of acceleration. It feels like the hand of God pushing me down the road.–Tim Pickens, Huntsville, AL